The problems associated with wax in lubricating oils are very well known. In the distillation of crude oil, a proportion of wax is present in cuts taken in the lubricating oil range. Some of the wax remains dissolved in the oil, whereas other fractions form a haze as the oil fraction ages at ambient temperatures. The appearance of haze affects the aesthetics and the economics of petroleum products. Products that are clear and bright are more highly valued than those that are hazy. The clear and bright test is a qualitative test for determining free water and particulate matter in oil and is, therefore, subject to human interpretation.
Haze manifests itself as a milky or cloudy appearance in the oil and is often caused by wax or by both wax and tiny water droplets being present in the oil. Typically a minimum amount of wax will cause some oils to look hazy. The haze precursors are wax type molecules which are more difficult to remove than are the waxes typically associated with pour point and cloud point.
In preparing a petroleum product such as a finished heavy lubricant base stock, the base stock will be subjected to a dehazing step to improve its appearance. Dehazing is typically achieved by either solvent or catalytic dewaxing to remove those constituents that result in haziness. Solvent dewaxing physically removes wax from oil as a solid at low temperature using a solvent. Whereas, catalytic dewaxing uses a catalyst that converts long chain normal or slightly branched long chain hydrocarbon (wax) into shorter chain hydrocarbon by cracking/fragmentation, to thereby reduce pour point and cloud point (both of which are measured at low temperature). However, the haze precursors of interest do not necessarily respond to conventional wax removal techniques such as solvent or catalytic dewaxing or would do so only with severe loss in yield of the desired product.
U.S. Pat. No. 6,579,441 is directed to a base oil feed having a reduced tendency to form haze at ambient or sub-ambient temperatures. The haze forming tendency of the oil is determined by measuring NTU. NTU has long been used to measure turbidity in liquids such as water. While U.S. Pat. No. 6,579,441 teaches a base oil having an NTU value of less than 2 with a reduced tendency to develop haze, the base oil does not remain clear and bright after being left standing at ambient conditions for an extended period of time.
Despite the advances in lubricant oil formulation technology, experience has shown that a lubricant base oil having a satisfactory cloud point, e.g., 5° C. or even lower, and pour point and that is clear and bright right after it is cooled to room temperature, may upon storage develop a haze. This phenomenon is referred to herein as delayed onset haze formation.
Thus, there remains a need for lubricant base oils that remain clear and bright even after being cooled to room temperature and stored for a period of time as long as several months, e.g., up to six months. The present invention provides for a heavy hydrocarbon composition having high viscosity, low pour and cloud points and a haze disappearance temperature of 20° C. (68° F.) or less, allowing the composition to remain clear and bright at room temperature.